The primary action of light stimulation on the electrical properties of rod type photoreceptors will be investigated using electrophysiological techniques. The study will be performed in the retina of the toad, Bufo Marinus. A retinal perfusion system has been developed which allows rapid changing of the ionic composition of the fluid bathing the photoreceptors. The effects of monovalent and divalent cations on the kinetics of the photoresponse will be investigated.